Functionalized Ruthenium–Phosphine Metal–Organic Framework for Continuous Vapor-Phase Dehydrogenation of Formic Acid

Metal–organic frameworks (MOFs) are ideal hosts for incorporation of molecular complexes without altering their original ligand environment; molecular catalysts can thus be easily synthesized and used in gas- and vapor-phase reactions operated in continuous mode. We report the immobilization of a molecular ruthenium complex in a phosphine-functionalized MOF that is highly efficient in the vapor-phase dehydrogenation of formic acid. The catalyst exhibited exclusive selectivity to hydrogen and carbon dioxide with outstanding stability at 145 °C (TON > 1 290 000). Our results represent a noteworthy improvement over heterogeneous ruthenium systems in terms of selectivity in the gas-phase, while reaching a productivity level higher than that of state-of-the-art homogeneous catalysts.ISSN:2155-543

Functionalized Ruthenium-Phosphine Metal-Organic Framework for Continuous Vapor-Phase Dehydrogenation of Formic Acid

Metal–organic frameworks (MOFs) are ideal hosts for incorporation of molecular complexes without altering their original ligand environment; molecular catalysts can thus be easily synthesized and used in gas- and vapor-phase reactions operated in continuous mode. We report the immobilization of a molecular ruthenium complex in a phosphine-functionalized MOF that is highly efficient in the vapor-phase dehydrogenation of formic acid. The catalyst exhibited exclusive selectivity to hydrogen and carbon dioxide with outstanding stability at 145 °C (TON > 1 290 000). Our results represent a noteworthy improvement over heterogeneous ruthenium systems in terms of selectivity in the gas-phase, while reaching a productivity level higher than that of state-of-the-art homogeneous catalysts.ISSN:2155-543

Role of Defects in Pore Formation in MFI Zeolites

Silicalite crystals, crystallized with fluoride ions in the synthesis mixture, are stable in alkaline solution for at least 1 week of base treatment. Nuclear magnetic reasonance (NMR) and infrared (IR) spectroscopy evidence that silicalite is essentially free of defect sites. MFI crystals with decreasing aluminum content in the order ZSM-5 (Si/Al = 14) > ZSM-5 (Si/Al = 50) > silicalite were base leached. Based on X-ray diffraction (XRD) patterns, only silicalite and ZSM-5 (Si/Al = 14) retain the MFI morphology for as long as 1 week in alkaline solution (0.1 M NaOH at 80 °C), while that of ZSM-5 with Si/Al = 50 is completely destroyed. The modulating effect of aluminum sites is excluded in silicalite, and its lack of defects explains the stability. Scanning electron microscopy (SEM) images show that leaching takes place in the center of the crystals preferentially in the [010] direction. The preferential pore formation indicates that structural properties have an effect on creation of porosity upon alkaline treatment

Liquid–Vapor Interface of Formic Acid Solutions in Salt Water: A Comparison of Macroscopic Surface Tension and Microscopic in Situ X-ray Photoelectron Spectroscopy Measurements

The liquid–vapor interface is difficult to access experimentally but is of interest from a theoretical and applied point of view and has particular importance in atmospheric aerosol chemistry. Here we examine the liquid–vapor interface for mixtures of water, sodium chloride, and formic acid, an abundant chemical in the atmosphere. We compare the results of surface tension and X-ray photoelectron spectroscopy (XPS) measurements over a wide range of formic acid concentrations. Surface tension measurements provide a macroscopic characterization of solutions ranging from 0 to 3 M sodium chloride and from 0 to over 0.5 mole fraction formic acid. Sodium chloride was found to be a weak salting out agent for formic acid with surface excess depending only slightly on salt concentration. In situ XPS provides a complementary molecular level description about the liquid–vapor interface. XPS measurements over an experimental probe depth of 51 Å gave the C 1s to O 1s ratio for both total oxygen and oxygen from water. XPS also provides detailed electronic structure information that is inaccessible by surface tension. Density functional theory calculations were performed to understand the observed shift in C 1s binding energies to lower values with increasing formic acid concentration. Part of the experimental −0.2 eV shift can be assigned to the solution composition changing from predominantly monomers of formic acid to a combination of monomers and dimers; however, the lack of an appropriate reference to calibrate the absolute BE scale at high formic acid mole fraction complicates the interpretation. Our data are consistent with surface tension measurements yielding a significantly more surface sensitive measurement than XPS due to the relatively weak propensity of formic acid for the interface. A simple model allowed us to replicate the XPS results under the assumption that the surface excess was contained in the top four angstroms of solution

Nanoparticle-induced charge redistribution of the air-water interface

The air–water interface is believed to carry a negative electrostatic potential that is nontrivial to invert through pH, electrolyte, or electrolyte strength. Here, through a combined experimental and theoretical study, we show that the close approach of a negatively charged nanoparticle induces a charge redistribution of the air–water interface. Using different electrolytes to control the interfacial potential of the nanoparticles, X-ray photoelectron spectroscopy (XPS) results establish that nanoparticles with a more negative zeta potential adsorb closer to the air–water interface than do the same particles with a less negative zeta potential. The short-ranged attractive force between two (nominally) negative surfaces is caused by charge redistribution under the strong electric field of the nanoparticle that locally inverts the charge density of the air–water interface from negative to positive. The nature of the nanoparticle’s counterions modulates the attractive interaction, which thus could be used to control reactivity, stability, and nanoparticle self-assembly at air–water interfaces.ISSN:1932-7455ISSN:1932-744

Changes in the Silanol Protonation State Measured In Situ at the Silica-Aqueous Interface

Recent nanomedical applications have again highlighted the significance of silica surface chemistry in solution. Here, we report in situ electronic structure measurements at the silica-aqueous interface as a function of pH for nanoparticles (NPs) of 7, 12, and 22 nm using a liquid microjet in combination with synchrotron radiation. The Si K-edge X-ray absorption near-edge spectroscopy (XANES) spectra reveal a change in shape of the Si 1s -> t(2) (Si 2p-3s) absorption brought about by changes in the silanol protonation state at the interface of the NPs as a result of changes in solution pH. Our results are consistent with the number of silanol groups changing the protonation state being inversely correlated with the SiO2 NP size. The importance of in situ studies is also demonstrated by comparing the XANES spectra of aqueous 7 nm SiO2 with the same dehydrated sample in vacuum

Non-uniform spatial distribution of tin oxide (SnO2) nanoparticles at the air-water interface

Depth resolved X-ray photoelectron spectroscopy (XPS) combined with a 25 μm liquid jet is used to quantify the spatial distribution of 3 nm SnO2 nanoparticles (NPs) from the air–water interface (AWI) into the suspension bulk. Results are consistent with those of a layer several nm thick at the AWI that is completely devoid of NPs.ISSN:1359-7345ISSN:1364-548

Liquid–Vapor Interface of Formic Acid Solutions in Salt Water: A Comparison of Macroscopic Surface Tension and Microscopic in Situ X‑ray Photoelectron Spectroscopy Measurements

The liquid–vapor interface is difficult to access experimentally but is of interest from a theoretical and applied point of view and has particular importance in atmospheric aerosol chemistry. Here we examine the liquid–vapor interface for mixtures of water, sodium chloride, and formic acid, an abundant chemical in the atmosphere. We compare the results of surface tension and X-ray photoelectron spectroscopy (XPS) measurements over a wide range of formic acid concentrations. Surface tension measurements provide a macroscopic characterization of solutions ranging from 0 to 3 M sodium chloride and from 0 to over 0.5 mole fraction formic acid. Sodium chloride was found to be a weak salting out agent for formic acid with surface excess depending only slightly on salt concentration. In situ XPS provides a complementary molecular level description about the liquid–vapor interface. XPS measurements over an experimental probe depth of 51 Å gave the C 1s to O 1s ratio for both total oxygen and oxygen from water. XPS also provides detailed electronic structure information that is inaccessible by surface tension. Density functional theory calculations were performed to understand the observed shift in C 1s binding energies to lower values with increasing formic acid concentration. Part of the experimental −0.2 eV shift can be assigned to the solution composition changing from predominantly monomers of formic acid to a combination of monomers and dimers; however, the lack of an appropriate reference to calibrate the absolute BE scale at high formic acid mole fraction complicates the interpretation. Our data are consistent with surface tension measurements yielding a significantly more surface sensitive measurement than XPS due to the relatively weak propensity of formic acid for the interface. A simple model allowed us to replicate the XPS results under the assumption that the surface excess was contained in the top four angstroms of solution